In a conventional compression type refrigerator, particularly a compression refrigerator for a car air conditioner, there has been used a variable displacement-type refrigeration apparatus provided with a variable mechanism in which an angle of a slanted plate is changed to adjust the stroke of a piston.
With such a system in which the stroke of the piston is controlled, however, an excess load is applied to the piston to cause wear or seizure thereof.
In recent years, variable speed compressor in which a traction drive mechanism is provided within the compressor to control the speed of a piston has been developed.
A lubricating oil composition for use in such a compression type refrigerator is required not only to have excellent lubricating properties and stability that have been demanded of a lubricating oil composition for the conventional refrigeration apparatus but also to exhibit suitable traction performance.
No lubricating oil compositions for a compression type refrigerator have been found, however, that have high traction performance, in particular that have a high traction coefficient in a refrigerant atmosphere, as well as excellent lubricating properties and stability.
There is, therefore, a great demand for a lubricating oil composition for a compression type refrigerator having a traction mechanism which is capable of showing excellent traction performance as well as excellent lubricating properties and stability.